1. Field of the Invention
This invention relates to power-efficient and low-cost methods for distributing power to SPD loads including SPD windows, SPD shades, and other window enhancements or other devices incorporating SPD film.
2. Description of the Related Art
Light valves have been known for more than seventy years for the modulation of light. As used herein, a light valve is defined as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon, usually in the form of transparent, electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an “activatable material”), which may be either a liquid suspension of particles, or a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension” or “a light valve suspension”) comprises small, anisometrically shaped particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles, and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Light valves of the type described herein are also known as “suspended particle devices” or “SPDs.”
Light valves have been proposed for use in numerous applications including, e.g., alphanumeric and graphic displays; television displays; filters for lamps, cameras, optical fibers, and windows, sunroofs, sunvisors, eyeglasses, goggles and mirrors and the like, to control the amount of light passing therethrough or reflected therefrom as the case may be. As used herein the term “light” generally refers to visible electromagnetic radiation, but where applicable, “light” can also comprise other types of electromagnetic radiation such as, but not limited to, infrared radiation.
For many applications, as would be well understood in the art, it is preferable for the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging, associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film, the particles are generally present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.
As used herein, the terms “SPD film” and “light valve film” mean at least one film or sheet comprising a suspension of particles used or intended for use by itself or with other components as part of a light valve. The light valve film or SPD film comprises either: (a) a suspension of particles dispersed throughout a continuous liquid phase enclosed within one or more rigid or flexible solid films or sheets, or (b) a discontinuous phase of a liquid comprising dispersed particles, the discontinuous phase being dispersed throughout a continuous phase of a rigid or flexible solid film or sheet. The light valve film or SPD film may also comprise one or more other layers such as, without limitation, a film, coating or sheet, or combination thereof, which may provide the light valve film or SPD film with, for example, (1) scratch resistance (2) protection from ultraviolet radiation (3) reflection of infrared energy, and/or (4) electrical conductivity for transmitting an applied electric or magnetic field to the activatable material.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is formed by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. Examples of these are described in U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention.
U.S. Pat. No. 6,804,040 B2 describes a method and device for controlling the ac voltage provided to a suspended particle device, hereafter called an SPD.
When the electric field is zero, the suspended particles are randomly oriented because of Brownian movement, and this randomness usually has the effect of reducing or blocking the passage of light. Upon application of an electric field, the particles align, usually with their long axes parallel to the electric field, which allows light to pass through the SPD load.
With currently available SPD films, a nominal AC voltage of 120 V can create a clear state, whereas the absence of voltage results in a dark state. Intermediate voltages produce light transmissions between the clear and dark states, that is, relative light transmission between 0 and 100 percent. The exact value of acceptable voltage required for a clear state depends on the thickness of the dielectric layer between conducting layers, the dielectric constant of the SPD emulsion, and the nature of the SPD particles. The function of an SPD controller is to produce an AC voltage between 0 and Vmax, the voltage level that produces a subjectively acceptable level of window clarity in a given application. Although voltages as low as 60 VAC may be acceptable in some applications, this discussion assumes 120 V rms as the standard value for Vmax because 120 VAC produces an acceptable clear state in all of today's SPD films. Future films may eventually operate with voltages as low as 10 VAC or lower, when the films become thinner or more efficient, for example.
The following is a brief description of liquid light valve suspensions known in the art which are useful in forming windows retrofitted with a switchable glazing, although the invention is not limited to the use of only such suspensions, nor is it limited to the use of switchable glazings per se since alternate classes of light modulating devices (e.g., non-switchable devices), as described below, can be used in place of such switchable window enhancements.
1. Liquid Suspending Media and Stabilizers
A liquid light valve suspension for use with the invention may be any liquid light valve suspension known in the art and may be formulated according to techniques well known to one skilled in the art. The term “liquid light valve suspension”, as used herein, means a “liquid suspending medium” in which a plurality of small particles is dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer, which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Liquid light valve suspensions useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general, at least one of the liquid suspending medium and the polymeric stabilizer dissolved therein is chosen in a manner known in the art so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, after which they are re-coated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
2. Particles
Inorganic and organic particles may be incorporated into a light valve suspension useful in forming a switchable enhancement for use in retrofitting a window in accordance with the present invention. Such particles may be either light-absorbing or light-reflecting in the visible portion of the electromagnetic spectrum. For some particular applications the particles can be reflective at infrared wavelengths.
Conventional SPD light valves have generally employed polyhalide particles of colloidal size, that is the particles generally have a largest dimension averaging about 1 micron or less. As used herein, the term “colloidal”, when referring to particle size, shall have the meaning given in the preceding sentence. Preferably, most polyhalide or other particles used or intended for use in an SPD light valve suspension used in accordance with the invention will have a largest dimension which averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low. As used herein, the term “anisometric”, which refers to particle shape, means that at least one dimension is larger than another. Typically, anisometric particles (sometimes referred to as particles which are anisometrically shaped), are desirable in an SPD light valve suspension so that the particles will block less light when the suspension is activated than when it is unactivated. For some suspensions the reverse is true, however. Desirable anisometric shapes for the particles include, without limitation thereto, particles shaped like rods, cylinders, plates, needles, blades, prisms, and other shapes known in the art.
A detailed review of prior art polyhalide particles is found in “The Optical Properties and Structure of Polyiodides” by D. A. Godina and G. P. Faerman, published in “The Journal of General Chemistry”, Vol. 20, pp. 1005-1016 (U.S.S.R. 1950).
Herapathite, for example, is defined as a quinine bisulfate polyiodide, and its formula is given under the heading “quinine iodsulfate” as 4C20H24N2O2.3H2SO4.2HI.I4.6H2O in The Merck Index, 10.sup.th Ed. (Merck & Co., Inc., Rahway, N.J.). In polyiodide compounds, the iodide anion is thought to form chains and the compounds are strong light polarizers. See U.S. Pat. No. 4,877,313 and Teitelbaum et al. JACS 100 1978), pp. 3215-3217. The term “polyhalide” is used herein to mean a compound such as a polyiodide, but wherein at least some of the iodide anion may be replaced by another halide anion. More recently, improved polyhalide particles for use in light valves have been proposed in U.S. Pat. Nos. 4,877,313, 5,002,701, 5,093,041 and 5,516,463. These “polyhalide particles” are formed by reacting organic compounds, usually containing nitrogen, with elemental iodine and a hydrohalide acid or an ammonium halide, alkali metal halide or alkaline earth metal halide.
For some applications, however, it may be desirable to use non-polyhalide particles in light valve suspensions and films, especially where the stability of the material composing the particles is known to be excellent.
3. Power Sources
Regardless of the type of SPD load, it is currently the practice to apply a high AC voltage to the SPD load to attain maximum clarity, that is, maximum light transmission through the SPD film. This applied voltage is typically 120 VAC at 60 Hz, which makes SPD loads ideal for use with ac power available in the United States. For those countries where 240 VAC at 50 Hz is common, a 2-to-1 step-down transformer may be used to obtain 120 VAC. There is no noticeable difference between 50 Hz and 60 Hz for running an SPD load.
U.S. Pat. No. 6,804,040 B2 describes an SPD controller, a method and device for controlling a high ac voltage provided to an SPD load. Basically, it modulates 120 VAC to produce an output that is adjustable manually, automatically, or by remote control to vary the voltage being applied to an SPD load from 0 to 120 VAC. The low end of the range (0 V) produces a dark state, that is, very little transmission of incident light. The high end of the range (120 VAC) produces a clear state, that is very high transmission of incident light. Intermediate voltages between 0 and 120 VAC produce intermediate light transmissions between the dark and clear states.
Turning initially to FIG. 1, there is shown a conventional distribution of AC power from a source 10 to SPD loads SPD1-SPD5 in architectural applications. In the United States, the line voltage has a nominal value of 120 VAC at 60 Hz. Although voltages less than 120 VAC may produce almost clear states with some SPD film types, 120 VAC is the ideal at present for SPD loads because it produces a fully clear state in all SPD films. In Europe, the nominal line voltage is 240 VAC at 50 Hz. In Europe, a 2-to-1 step-down transformer is used to obtain the desired maximum voltage of 120 VAC.
In FIG. 1, the use of five SPD loads is shown, for illustration only. The actual number of SPD loads will depend on the size of the architectural structure, the total window area involved, the number of individual SPD controllers being used, and other factors. Although five SPD loads will be shown in the figures herein, a much larger number of SPD loads may be present in various embodiments of the invention. In some large office buildings, hundreds or even thousands of SPD loads may be present.
To understand some of the problems involved in power distribution, we need to discuss the power requirements of SPD loads. To begin with, SPD film capacitance may vary from 40 nF per square foot (hereafter abbreviated sf) to 90 nF per sf. The former capacitance is for the darkest film type currently available and the latter is for the lightest film type. The corresponding capacitive reactances for the foregoing capacitances are approximately 66 kilohms/sf and 30 kilohms/sf. At 120 VAC and 60 Hz, these impedances result in ac currents of 1.8 mA/sf and 4 mA/sf. These data are summarized below for later reference:
Input: 120 VAC at 60 Hz
40 nF film: 1.8 mA/sf
90 nF film: 4 mA/sf
As an example of calculating required SPD load current, assume an architectural structure of 3000 square feet with a 20 percent window area of 600 square feet of 90 nF film. Then, the total SPD load current isI=(600 sf)(4 mA/sf)=2.4 AIf 40 nF film is used instead of 90 nF film, the total SPD current decreases toI=(600 sf)(1.8 mA/sf)=1.08 A
The wiring that runs throughout the structure of FIG. 1 must conform with the United States National Electrical Code (NEC). Because of the high ac voltage being distributed, the National Electrical Code addresses a number of issues such as prevention of electric shock, fire hazard, etc. It is the intent of this invention to mitigate some of these issues by distributing power to SPD loads in a safer and less costly manner.